<!DOCTYPE html>
<meta charset="UTF-8">
<style>

  .node circle {
    fill: #fff;
    stroke: steelblue;
    stroke-width: 3px;
  }

  .node text {
    font: 12px sans-serif;
  }

  .link {
    fill: none;
    stroke: #ccc;
    stroke-width: 2px;
  }

</style>

<body>

<!-- load the d3.js library -->
<script src="https://d3js.org/d3.v5.min.js"></script>
<script>

  var treeData =
    {
      "name": "Top Level",
      "children": [
        {
          "name": "Level 2: A",
          "children": [
            { "name": "Son of A" },
            { "name": "Daughter of A" }
          ]
        },
        { "name": "Level 2: B" }
      ]
    };

  // Set the dimensions and margins of the diagram
  var margin = {top: 20, right: 90, bottom: 30, left: 90},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

  // append the svg object to the body of the page
  // appends a 'group' element to 'svg'
  // moves the 'group' element to the top left margin
  var svg = d3.select("body").append("svg")
    .attr("width", width + margin.right + margin.left)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform", "translate("
      + margin.left + "," + margin.top + ")");

  var i = 0,
    duration = 750,
    root;

  // declares a tree layout and assigns the size
  var treemap = d3.tree().size([height, width]);

  // Assigns parent, children, height, depth
  root = d3.hierarchy(treeData, function(d) { return d.children; });
  root.x0 = height / 2;
  root.y0 = 0;

  // Collapse after the second level
  root.children.forEach(collapse);

  update(root);

  // Collapse the node and all it's children
  function collapse(d) {
    if(d.children) {
      d._children = d.children
      d._children.forEach(collapse)
      d.children = null
    }
  }

  function update(source) {

    // Assigns the x and y position for the nodes
    var treeData = treemap(root);

    // Compute the new tree layout.
    var nodes = treeData.descendants(),
      links = treeData.descendants().slice(1);

    // Normalize for fixed-depth.
    nodes.forEach(function(d){ d.y = d.depth * 180});

    // ****************** Nodes section ***************************

    // Update the nodes...
    var node = svg.selectAll('g.node')
      .data(nodes, function(d) {return d.id || (d.id = ++i); });

    // Enter any new modes at the parent's previous position.
    var nodeEnter = node.enter().append('g')
      .attr('class', 'node')
      .attr("transform", function(d) {
        return "translate(" + source.y0 + "," + source.x0 + ")";
      })
      .on('click', click);

    // Add Circle for the nodes
    nodeEnter.append('circle')
      .attr('class', 'node')
      .attr('r', 1e-6)
      .style("fill", function(d) {
        return d._children ? "lightsteelblue" : "#fff";
      });

    // Add labels for the nodes
    nodeEnter.append('text')
      .attr("dy", ".35em")
      .attr("x", function(d) {
        return d.children || d._children ? -13 : 13;
      })
      .attr("text-anchor", function(d) {
        return d.children || d._children ? "end" : "start";
      })
      .text(function(d) { return d.data.name; });

    // UPDATE
    var nodeUpdate = nodeEnter.merge(node);

    // Transition to the proper position for the node
    nodeUpdate.transition()
      .duration(duration)
      .attr("transform", function(d) {
        return "translate(" + d.y + "," + d.x + ")";
      });

    // Update the node attributes and style
    nodeUpdate.select('circle.node')
      .attr('r', 10)
      .style("fill", function(d) {
        return d._children ? "lightsteelblue" : "#fff";
      })
      .attr('cursor', 'pointer');


    // Remove any exiting nodes
    var nodeExit = node.exit().transition()
      .duration(duration)
      .attr("transform", function(d) {
        return "translate(" + source.y + "," + source.x + ")";
      })
      .remove();

    // On exit reduce the node circles size to 0
    nodeExit.select('circle')
      .attr('r', 1e-6);

    // On exit reduce the opacity of text labels
    nodeExit.select('text')
      .style('fill-opacity', 1e-6);

    // ****************** links section ***************************

    // Update the links...
    var link = svg.selectAll('path.link')
      .data(links, function(d) { return d.id; });

    // Enter any new links at the parent's previous position.
    var linkEnter = link.enter().insert('path', "g")
      .attr("class", "link")
      .attr('d', function(d){
        var o = {x: source.x0, y: source.y0}
        return diagonal(o, o)
      });

    // UPDATE
    var linkUpdate = linkEnter.merge(link);

    // Transition back to the parent element position
    linkUpdate.transition()
      .duration(duration)
      .attr('d', function(d){ return diagonal(d, d.parent) });

    // Remove any exiting links
    var linkExit = link.exit().transition()
      .duration(duration)
      .attr('d', function(d) {
        var o = {x: source.x, y: source.y}
        return diagonal(o, o)
      })
      .remove();

    // Store the old positions for transition.
    nodes.forEach(function(d){
      d.x0 = d.x;
      d.y0 = d.y;
    });

    // Creates a curved (diagonal) path from parent to the child nodes
    function diagonal(s, d) {

      path = `M ${s.y} ${s.x}
            C ${(s.y + d.y) / 2} ${s.x},
              ${(s.y + d.y) / 2} ${d.x},
              ${d.y} ${d.x}`

      return path
    }

    // Toggle children on click.
    function click(d) {
      if (d.children) {
        d._children = d.children;
        d.children = null;
      } else {
        d.children = d._children;
        d._children = null;
      }
      update(d);
    }
  }

</script>
</body>
